window.onload = function() {
    var trigger = document.getElementById('hamburger'),
        classList = trigger.classList,
        isClose = true;
    
    trigger.onclick = function() {
        burgerTime();
    }

    function burgerTime() {
        if(isClose == true) {
            classList.remove('is-open');
            classList.add('is-closed');
            isClose = false;
        } else {
            classList.remove('is-closed');
            classList.add('is-open');
            isClose = true;
        }
    }
}